The present invention relates to subsurface waste water disposal, in particular to the treatment of waste water in a leach field.
In a waste water treatment system of the type commonly used for domestic dwellings and other limited volume waste water sources, waste water is flowed first into a tank, where it is acted upon by microorganisms in an anaerobic environment. The temperature in the septic tank, and of the effluent from the tank, is a combination of the temperature of the input waste water, the septic tank environment, and any heat generated by biochemical activity in the tank. The input waste water temperature is of course a function of the source water temperature and any heat added during passage through the dwelling or other waste water generator. There is typically a net heat added to the source water because, among other things, hot water used washing persons and things is dumped into the sewer. A typical septic tank is designed so that the average residence time of waste water is 1-3 days. The heat of the typical anaerobic reaction in the septic tank is considered minor in the view of the volume of in-flowing waste water. And, there is tendency for waste water to equilibrate with the temperature of the surrounding soil or other septic tank environment. Nonetheless, it can be expected and is found that the average temperature of the effluent from a septic tank will be somewhat higher than the average surrounding soil temperature. For information about typical temperature variation in wastewater, see Part 3-2 of the book by Metcalf and Eddy, Inc., xe2x80x9cWaste Water Engineeringxe2x80x9d 3rd Ed. McGraw Hill (1991).
Waste water effluent from the septic tank is typically flowed into a secondary waste water treatment system where it can be acted upon biochemically in an aerobic environment, to thereby be made relatively benign and purified, and to then be delivered back into the water table of the earth or another reservoir, and to thereafter be re-used. Most commonly, waste water will be flowed into a leach field. A leach field is comprised of a one or more lengths of conduits which are buried beneath the surface of the earth, and water flows from the conduits laterally and vertically into the surrounding soil. A traditional type of leach field is comprised of perforated pipe running along within trenches filled with crushed stone or gravel overlaid by soil. Less common and less preferred is a covered leach pit, sometimes called a dry well. Concrete chambers with perforated walls, called galleries, are used. In recent years, it has been popular to construct leach fields comprised of lengths of interconnected arch shaped plastic chambers. Generally, the foregoing, and any other structure or device which is buried in the soil which has the purpose to hold or convey waste water, including those which receive waste water which has not first passed through a septic tank, so it can infiltrate into the soil, is referred to here as a conduit.
The soil adjacent to a leach field conduit comprises a region called the influence zone. Waste water from the conduits is biochemically acted upon as it flows through the influence zone to other regions within the soil. The temperature within the influence zone during waste water treatment is a function of various factors, including the ambient temperature of the soil, the atmospheric conditions above the soil surface, the input temperature of the wastewater, and the any heat generated or added in the influence zone. As just discussed, the effluent waste water from a septic tank will tend to be close to, but warmer than, the soil temperature. Heat loss during flow to and through the leaching system conduits will tend to equilibrate the waste water with the soil. Any heat of aerobic reaction in the influence zone, which is not known to be significant, will tend to raise temperature. The typical net result is that on average the temperature in the influence zone can be expected to be above the temperature in like soil remote from the leaching system, principally attributable to the heat content of the waste water from the septic tank source.
The temperature in soil in general and in the influence zone in particular can be affected by local climatic conditions or seasonal changes of the atmosphere. Deep beneath the level of any ordinary leach field, there is rock or subsoil which is reflective of the average annual temperature. The soil near the surface containing a typical leach field is substantially warmed in the summer and cooled in the winter relative to the subsoil temperature; and, the influence zone of a leach field will be accordingly affected. When the influence zone soil is cool, biochemical activity, and thus treatment of waste water, can be adversely affected. When proper biochemical treatment does not take place, organic substances which are pollutants from the wastewater can accumulate in the soil of the leach field. That accumulation will tend to adversely affect the performance of the leach field in the short or long run. A leach field system may fail at certain times of the year, or after a period of use, even years. As a corollary, a leach field may have to be initially made larger than would at first appear to be necessary, because there will be adverse low temperatures in the soil at certain times.
Thus, there is a need for technology which can stabilize and improve the operation of existing fields, and which can counter adverse environmental temperature conditions.
An object of the invention is to increase the rapidity and extent of treatment within a leach field of a waste water system. A further object is to increase biochemical activity within the influence zone of a leach field system, and to thereby either restore or enhance the capacity of a leach field.
In accord with the invention, the soil within the influence zone of a waste water system comprised of leach field conduits is heated, to significantly affect the biochemical activity which is expected to make the waste water environmentally benign. Different means are employed for adding heat in different embodiments of the invention. In one embodiment, heated air is flowed into the conduits and then into the soil which comprises the influence zone. In another embodiment, heating elements, such as resistance heaters or tubes carrying heated fluid, are placed in or in vicinity of the influence zone soil. For example, heating elements are placed adjacent to, and or beneath, and or within the conduits. In another embodiment, heated gas is delivered directly into the influence zone, as by perforated pressurized pipes.
The effectiveness of the heat addition to the influence zone is enhanced by the use of insulation directly above the conduits or on the surface of the soil, to inhibit loss of heat from vicinity of the conduit through the soil surface.
In a preferred practice of the invention, the temperature of at least a portion of the influence zone is raised by at least 1-2xc2x0 F., preferably more substantially by about 5xc2x0 F. or more, compared to what the temperature would be in absence of use of the invention, to enhance biochemical activity and waste water treatment. A 1-2xc2x0 F. change is significant to biochemical activity. Preferably, the temperature in the influence zone will be raised or maintained in the range of about 50-100xc2x0 F.; more preferably in the range 50-75xc2x0 F., with a maximum of about 120xc2x0 F. In another practice of the invention, air which is warmed by an existing source, such as the atmosphere or a heated space in a dwelling, is flowed to the leach field, to heat the field, whenever the soil temperature is less than the source temperature.
The heat source for heating elements or air or other fluid flowed into the leach field may be any of familiar sources, such as fuel combustion, electric resistance heating, and the like. Air flowed into the leach field by a blower type air mover may be heated by purposeful inefficiency of the blower. Heat for the leach field may be extracted from a geothermal source, for example, the natural water table in the earth. The extracted heat is delivered to the influence zone by means of a circulating heat exchange liquid or the working substance (refrigerant) or a heat pump system. In another aspect of the invention, air is flowed from a source with a temperature which has a certain found heat content, such as atmospheric air or air from a heated basement, according to whether the source air temperature exceeds the soil temperature.
Heating may be continuous or intermittent. When the invention is used, the capacity of an existing system will be enhanced; and systems that are failed can be restored. As a corollary, the leach field size of a new system might be smaller than normal waste water engineering would dictate, so problem sites can be better used.
The foregoing and other objects, features and advantages of the present invention will become more apparent from the following description of preferred embodiments and accompanying drawings.